1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus for forming an image on both surfaces of a transfer material.
2. Description of the Related Art
A configuration of a conventional image forming apparatus will be described hereinafter.
<Configuration of Color Image Forming Apparatus>
FIG. 8 is a schematic entire configuration diagram illustrating one example of a laser printer as a color image forming apparatus. The image forming apparatus forms an electrostatic latent image by an image light which is formed based on an image signal transmitted from a controller part not illustrated in an image forming part (image forming unit) as illustrated in FIG. 8, and develops the electrostatic latent image and superimposes and transfers a visible image to form a color visible image. The image forming apparatus transfers the color visible image onto a transfer material and fixes the color visible image on the transfer material. The image forming part is configured by photosensitive drums 5Y, 5M, 5C and 5K as a plurality of image bearing members for development colors arranged in parallel at each station, and charging devices 7Y, 7M, 7C and 7K as primary charge members. Further, the image forming part is configured by developing devices 8Y, 8M, 8C and 8K as developing means, an intermediate transfer belt 12 as an intermediate transfer member, a transfer part (transfer unit) to be a secondary transfer position 13, a fixing device 14 (fixing unit), a feeding sheet roller 19 and a registration roller 20.
The photosensitive drums 5Y, 5M, 5C and 5K, the charging devices 7Y, 7M, 7C and 7K, and the developing devices 8Y, 8M, 8C and 8K are loaded on toner cartridges 10Y, 10M, 10C and 10K which are attachable to and detachable from an image forming apparatus main body.
The photosensitive drums 5Y, 5M, 5C and 5K are formed by coating outer peripheries of aluminum cylinders with an organic optical conductive layer, and rotate by a drive force of a drive motor not illustrated being transmitted to the photosensitive drums. The drive motor rotates the photosensitive drums 5Y, 5M, 5C and 5K in a counter-clockwise direction in correspondence with an image forming operation. Exposure light to the photosensitive drums 5Y, 5M, 5C and 5K are configured to be sent from scanner parts 9Y, 9M, 9C and 9K, and selectively expose surfaces (on the image bearing members) of the photosensitive drums 5Y, 5M, 5C and 5K to thereby form an electrostatic latent image.
The primary charge member is configured by including the four electric charging devices 7Y, 7M, 7C and 7K for charging the photosensitive drums of yellow (Y), magenta (M), cyan (C) and black (K) at each station. The respective charging devices 7Y, 7M, 7C and 7K include sleeves 7YS, 7MS, 7CS and 7KS.
The developing means has a configuration including four developing devices 8Y, 8M, 8C and 8K which perform development of yellow (Y), magenta (M), cyan (C) and black (K) at each station to visualize an electrostatic latent image. The developing devices 8Y, 8M, 8C and 8K are provided with sleeves 8YS, 8MS, 8CS and 8CK, respectively.
The intermediate transfer belt 12 is in contact with the photosensitive drums 5Y, 5M, 5C and 5K, and rotates in a clockwise direction with rotation of the photosensitive drums 5Y, 5M, 5C and 5K at the time of color image formation to be subjected to transfer (primary transfer) of a visible image. Further, by holding and conveying a transfer material, the intermediate transfer belt 12 superimposes and transfers (multiple-transfers) the transfer material and a color visible image (multicolor toner image) on the intermediate transfer belt 12 at the same time.
A transfer part to be a secondary transfer position 13 is in contact with the intermediate transfer belt 12. The transfer part rotates in a counterclockwise direction with rotation of the intermediate transfer belt 12, and transfers (secondary transfer) the color visible image on the intermediate transfer belt 12 (on the intermediate transfer member) to the transfer material conveyed from the registration roller 20.
A feeding sheet sensor 21 detects whether a transfer material 2 which is fed from a sheet feeding cassette 1 reaches the feeding sheet sensor 21 within a predetermined time. A conveyance failure of the transfer material 2 at the time of sheet feeding can be detected from the detection result. A registration sensor 22 is provided for synchronizing the image formed on the intermediate transfer belt 12 and the transfer material 2 which is fed. The transfer material 2 is temporality stopped at the timing when a leading end of the transfer material 2 is sensed by the registration sensor 22, and the timing at which the transfer material 2 is conveyed again is controlled so that the image formed on the intermediate transfer belt 12 is transferred to a predetermined position of the transfer material 2. A prefixing sensor 23 and a fixing and discharging sensor 24 are also provided. Further, the fixing device 14 has a pressure roller 16 and a fixing roller 15, and a heater 17 is provided in the fixing roller. A thermistor 18 detects the surface temperature of the fixing roller 15, and energization to the heater 17 is controlled based on the detection result. A discharging sheet roller 25 discharges the transfer material 2. Further, a reversing flapper 26 switches a conveyance path of the transfer material 2 to form images on both surfaces of the transfer material 2.
<System Configuration of Image Forming Apparatus>
FIG. 9 is a block diagram for describing a schematic system configuration of a laser printer as an image forming apparatus. A controller control part 201 can mutually communicate with a host computer 200 and an engine control part 202 (220, 222 of FIG. 9).
The controller control part 201 receives image information and printing conditions from the host computer 200. The controller control part 201 transmits a printing reservation command for making reservation of a printing operation with printing information of each transfer material being added to the engine control part 202 based on the received printing conditions, and analyzes the received image information and converts the image information into bit data. Here, the printing information of each transfer material includes, for example, a sheet feeding port (sheet feeding cassette), a transfer material size, a printing mode and the like. The controller control part 201 transmits a printing start command for instructing start of printing operation to the engine control part 202 at a point of time when analysis of the image information is finished. When receiving the printing start command, the engine control part 202 outputs a /TOP signal (221 of FIG. 9) to be a reference timing of output of the video signal for a first station which is an image forming part of yellow. Subsequently, the engine control part 202 starts a sheet feeding operation, and causes the fed transfer material 2 to be temporarily on standby with the registration roller 20. Thereafter, the engine control part 202 feeds the transfer material 2 again from the registration roller 20 in synchronism with the toner image formed on the intermediate transfer belt 12 reaching the secondary transfer position 13. The engine control part 202 includes a video interface part 210, a CPU 211, an image processing GA 212, an image control part 213, a fixing control part 214, a sheet conveyance part 215 and a drive control part 216.
<Timing Chart of Both-Surface Printing>
FIG. 10 shows a timing chart of a case of realizing the highest throughput in both-surface printing (both-surface printing). The timing chart illustrates the /TOP signal (/TOP), drive of the registration roller 20 (registration roller drive), pick-up operation of the transfer material 2, a detection signal and reversing operation by the registration sensor 22 (reversing unit), and a detection signal by the fixing and discharging sensor 24. Further, in correspondence with each point of time illustrated by the broken line on the timing chart, the state of the image forming apparatus is illustrated in the order of (a) to (e) in FIG. 10. The throughput means the image formation number (the number of printing sheets) per unit time, and in order to realize the highest throughput, a time interval (see 300 of FIG. 10) between output of the /TOP signal of the first surface and output of the /TOP signal of the second surface needs to be the shortest.
On receiving a printing start command from the controller control part 201, the engine control part 202 prepares for printing, and outputs the /TOP signal of the first surface (311) when the preparation is made ((a) of FIG. 10), and starts a sheet feeding operation of the transfer material 2 (312). The engine control part 202 temporarily stops conveyance of the transfer material 2 (314) at a point of time (313) when the transfer material 2 which is picked up reaches the registration roller 20 where the registration sensor 22 is arranged. The engine control part 202 restarts (315) conveyance of the transfer material 2 in synchronism with a toner image t1 formed on the intermediate transfer belt 12 ((b) of FIG. 10), and transfers the toner image t1 onto the transfer material 2. The engine control part 202 thermally fixes the toner image t1 onto the transfer material 2 by the fixing device 14.
Thereafter, the engine control part 202 transfers the transfer material 2 to the position where a rear end of the transfer material 2 comes out of the fixing and discharging sensor 24 and the transfer material 2 can be reversed at 316 ((c) of FIG. 10), and starts the reversing operation by turning on (ON) the reversing flapper 26 and a reversing solenoid not illustrated (317) ((d) of FIG. 10). Next, the conveyance path is switched to the both-surface path by the reversing flapper 26 and the transfer material 2 is conveyed to the both-surface path. The engine control part 202 temporarily stops (324) conveyance of the transfer material 2 at a point of time (323) when the reversed transfer material 2 reaches the registration roller 20. Subsequently, the engine control part 202 restarts (325) conveyance of the transfer material 2 ((e) of FIG. 10) in synchronism with a toner image t2 of the second surface formed on the intermediate transfer belt 12, and transfers the toner image t2 onto the transfer material 2.
An output timing (321) of the /TOP signal of the second surface for realizing the highest throughput is determined with the following time as the reference based on the transfer material size designated to the engine control part 202 by the controller control part 201. More specifically, the output timing is determined by back calculation with the time in which the toner image t1 of the first surface is transferred onto the transfer material 2 at the secondary transfer position 13, and the transfer material 2 is reversed (317) and reaches the registration roller 20 again (323) as the reference.
As described above, the output timing of the /TOP signal of the second surface is determined by back calculation from the timing in which the second surface of the transfer material is reversed and reaches the registration roller 20 based on the transfer material size designated by the controller control part 201. Therefore, when the size in the conveying direction of the transfer material 2 which is actually set at the sheet feeding cassette 1 which is the sheet feeding port is larger than the transfer material size designated by the controller control part 201, the timing at which the transfer material 2 is reversed is later than the assumed timing. As a result, the transfer material 2 does not reach the registration roller 20 by the timing at which the toner image t2 of the second surface formed on the intermediate transfer belt 12 reaches the secondary transfer position 13, and detention of the transfer material 2, and sheet jamming (jam) occur.
Therefore, for example, Japanese Patent Application Laid-open Nos. H10-194529 and 2007-065411 avoid occurrence of jam as follows. Specifically, the size (length) in the conveying direction of the transfer material fed to the first surface is detected by using a sensor on the transfer material conveyance path. When the detected size in the conveying direction of the transfer material and the designated size in the conveying direction of the transfer material do not match with each other, a print error due to sheet size mismatch, which means a failure in printing, is output. The transfer material is discharged outside the apparatus, and printing operation is stopped, whereby removal processing of the transfer material by a user which is necessary when it is jammed is made unnecessary.
However, in each of the apparatuses of the conventional examples described above, if both-surface printing is carried out when the size in the conveying direction of the transfer material which is fed is larger than the designated size, a printing error due to mismatch of sheet size occurs. Therefore, when the error is determined, the toner image of the second surface remains on the intermediate transfer belt 12, and therefore, the toner is wasted. Further, as compared with the case of normal termination, much time is required for an operation of recovering the wasted toner.